Wireless communication systems are widely used to provide various types of communications. For example, voices and/or data are provided by the wireless communication systems. General wireless communication systems provide multiple users with one or more shared resources. For example, the wireless communication systems may use a variety of multiple access techniques such as Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), and Frequency Division Multiple Access (FDMA).
Orthogonal Frequency Division Multiplexing (OFDM) uses a plurality of orthogonal subcarriers. OFDM uses the characteristic of orthogonality between an Inverse Fast Fourier Transform (IFFT) and a Fast Fourier Transform (FFT). A transmitter transmits data after performing IFFT. A receiver restores original data by performing FFT on a received signal. The transmitter uses IFFT to combine multiple subcarriers, and the receiver uses corresponding FFT to separate the multiple subcarriers. According to OFDM, complexity of the receiver may be lowered in a frequency selective fading environment of wideband channels, and spectral efficiency may be enhanced through selective scheduling or the like in a frequency domain by utilizing different channel characteristics of subcarriers. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme based on OFDM. According to OFDMA, efficiency of radio resources may be enhanced by assigning different subcarriers to multiple users.
Recently, Multiple Input Multiple Output (MIMO) systems are spotlighted in order to maximize performance and communication capacity of wireless communication systems. The MIMO technique is a method that can improve transmission efficiency of transmit/receive data by employing multiple transmission antennas and multiple receive antennas, getting out of using one transmission antenna and one receive antenna used up to the present. A MIMO system is also referred to as a multiple antenna system. The MIMO technique does not depend on a single antenna path in order to receive one whole message, but applies a technique that gathers fragmented data segments received through a plurality of antennas and completes a message. As a result, data rate may be improved within a specific range, or a system range may be increased for a specific data rate.
Hereinafter, downlink means transmission from a base station (BS) to a user equipment (UE), and uplink means transmission from the UE to the BS.
Generally, the BS schedules radio resources of uplink and downlink in a wireless communication system. Feedback data are used to scheduling downlink radio resources in a multiple antenna system. The UE measures downlink channel and report the feedback data to the BS. The feedback data include a channel quality indicator (CQI), a rank indicator (RI) and a precoding matrix indicator (PMI). The CQI represents downlink channel condition, the RI corresponds to the number of useful transmission layers (or codewords) and the PMI denotes the index of a precoding matrix selected from a codebook. The UE transmits the feedback data to the BS, and the BS schedules downlink radio resources based on the feedback data.
In the multiple antenna system, the amount of the feedback data may be huge. Assuming that there are 10 subbands and 5 bit CQI for each subband is fed back, radio resources for total 50 bit is needed. Considering the PMI and the RI, more radio resources for the feedback data is needed. Furthermore, in rapidly changing channel environments, feedback data needs to be further frequently transmitted. Frequent transmission of the feedback data under limited radio resources may cause much overhead.
A method for reducing overhead due to transmission of the feedback data is required.